Scavengers, detritivores, and herbivores - Fishes as predators

Many fishes scavenge on dead and dying animals. A few species obtain most of their nutrition through scavenging.

Scavengers,
detritivores, and herbivores

Many
fishes scavenge on dead and dying animals. A few species obtain most of
their nutrition through scavenging (e.g., hagfishes) or detritivory (e.g., some
minnows and suckers, curimatids, prochilodontids, mullets, some Old World
cichlids), whereas others supplement predation and omnivory with scavenging
(e.g., catfishes, anguillid eels). Importantly, most predators will not pass up
freshly dead prey (otherwise bait would not work in hook-and-line fisheries)
and most scavengers and herbivores will take advantage of easily captured live
prey. In essence, although dietary specializations certainly exist, fishes are
highly opportunistic and will eat available prey of the appropriate size. At
Johnston Atoll in the tropical Pacific, discarded doughnuts are eaten readily
at the surface by such carnivores as snake eels, butterfl yfishes, and flounders,
and by such herbivores as damselfishes, parrotfishes, and surgeonfishes (D. A.
Mann, pers. comm).

For
scavenging animals, the predation cycle is usually shortened to search, wait,
manipulate, and handle, whereas for detritivores and herbivores the waiting is
eliminated. The major task befalling detritivores is one of separating
edible, fine particulate organic matter from any refractory, inedible sediments
ingested. Ridges in the mouth and a maze of passageways associated with the
gill rakers and epibranchial organs accomplish this in characoids. A winnowing
process occurs in the orobranchial chambers as fishes pick up a mouthful of
bottom material, sift it in the mouth, and expel inedible sediments back out
the mouth or out the gill openings. Detritivores have some of the longest or
most complexly folded intestines of any fishes, attesting to the resistance of
detritus to enzymatic digestion (Bowen 1983).

Herbivory
occurs less commonly
in fishes when compared to mammals and birds. Non-teleostean fishes are
exclusively carnivorous, with the possible exception of limited herbivory in
the Australian Lungfish, Neoceratodus forsteri. In teleosts we find the
evolution of pharyngeal mills and gizzards – mechanisms for
rupturing cell walls and digesting plant matter. The most diverse freshwater
fish taxa include substantial numbers of herbivorous species (characoids,
minnows, catfishes, cichlids), and herbivores on coral reefs are among the most
abundant fishes there (e.g., halfbeaks, parrotfishes, blennies, surgeonfishes,
rabbitfishes). Temperate waters are relatively lacking in herbivores, although
some marine families (porgies, sea chubs, Aplodactylidae, Odacidae,
pricklebacks) feed heavily on plant matter (Horn 1989).

Herbivory
requires accurate search and efficient handling. Herbivores, particularly those
that browse on upright macroalgae and do not graze on finer algal turfs, appear
to use visual cues for selecting edible versus inedible species. Herbivory is
consequently a primarily daytime activity. Targeted search is necessary because
plants defend themselves by being tough or by producing chemicals, often in the
form of halogenated terpenoids. Herbivorous fishes show strong preferences
among algal types, feeding preferentially on species that lack structural and
chemical defenses, while avoiding limestone-encrusted species or algae that
contain deterrent chemicals. Some of these chemicals can slow growth or cause
death in fishes (Horn 1989; Hay 1991).

Specializations
for handling plants relate to the difficulty with which cell walls are
disrupted, cellulose is digested, or defensive structures and chemicals are
overcome. Herbivorous fishes typically have long guts, high ingestion rates,
and rapid gut transit times. Large quantities of plant matter are passed
through the gut and relatively little nutrition is assimilated from each
ingested fraction. Cell walls are broken down in pharyngeal mills or lyzed in
highly acidic (pH as low as 1.5) stomachs, although con- clusive evidence of
enzymes capable of digesting cellulose (i.e., cellulase) is lacking. Unlike
insects and many herbivorous vertebrates, fishes also generally lack
endosymbiotic bacteria and other microbes that aid in the digestion of plant
matter. The exceptions include surgeonfishes, which contain bacteria, flagellates,
and peculiar protist-like organisms, and sea chubs (Kyphosidae), which possess
a unique digestive tract morphology and a hindgut microfl ora that aids in
digestive fermentation (Fishelson et al. 1985; Rimmer & Wiebe 1987). Interestingly,
some sea chubs feed heavily on brown algae that are avoided by most other
herbivores (Horn 1989; Kramer & Bryant 1995).

Herbivory
on coral reefs is intimately linked to both shoaling and territoriality. Most
herbivores either defend exclusive territories (e.g., damselfishes, adult
parrotfishes, blennies, surgeonfishes) or roam about the reef in monospecific
or heterospecific shoals (sea chubs, parrotfishes, surgeonfishes,
rabbitfishes). Territorial defense is very successful against solitary foragers
but less so against grouped foragers. Individuals in large groups sustain fewer
territorial attacks and have higher feeding rates than solitary foragers or
members of small groups. Hence territoriality by some fishes promotes
aggregation behavior in others (Robertson et al. 1976; Foster 1985).